The present invention generally relates to machines for drawing metal wire, for example, those used to manufacture wires to be bunched into conductors for flexible insulated cords, and more particularly to a wire drawing apparatus and method which facilitate changeovers to provide different wire reductions.
Wire drawing is an operation which is carried out in several passes, i.e. by passing the wire through a series of dies, the diameter of each of which is smaller than that of the preceding die. The wire is drawn through the dies by drawing capstans commonly referred to as "drawing blocks," the peripheral speeds of which increase progressively as the wire moves forward.
In one system of wire drawing which is used at present and is known as "wet wire drawing," the dies and, in some cases, the drawing blocks, are sprayed or immersed in a lubricating solution. Wire drawing machines can be divided into two classes, namely, in the single wire class which is more widely used at present, and the multi-wire class, the use of which is on the increase.
Wire drawing machines can also be divided into two different groups depending on the types of wire drawing blocks that are used. A first group of machines called "cone-type" wire drawing machines are characterized by drawing blocks of different diameters which are securely mounted on one and the same shaft to form a stepped cylinder or cone. The wire is looped around two sets of cones carried by a pair of spaced and substantially parallel shafts, and the dies are located in a die holder positioned in the path of the runs of wire between the two sets of cones. Wire drawing machines may comprise several pairs of cones, for example, two. This type of wire drawing machine offer the advantage of being very compact. On the other hand, the wire having the greatest diameter passes over the block having the smallest diameter, (i.e. the block providing the lowest speed). For certain types of applications this might not be satisfactory.
A second group of wire drawing machines have independent drawing blocks each mounted on a separate shaft. These machines are usually called "tandem" machines. The number of blocks is usually equal to the number of dies, with each die being upstream of its associated drawing block. The blocks, which in most applications have the same diameter, are driven at different speeds and the surface speed matches as closely as possible the difference in elongation between each die. The blocks may be positioned in many different arrangements, such as in-line, in a circle, along a spiral, etc. Generally, the wire can be strung along these blocks more easily than in the cone machines, but the drives are more complicated since a large number of blocks must all be driven at different speeds.
Multi-wire machines include both cone types and tandem type machines. Multi-wire machines with many configurations of cone and tandem blocks have been used in the art and they allow the drawing of several wires at the same time. At present eight wire machines are the most common.
All wire drawing machines are designed in such a way as to provide a pre-set difference in surface speed between successive blocks and this speed difference becomes a fixed parameter of the machine. The differences in surface speeds dictate the maximum reduction in area and the relative elongation of the wire from one block to the following one. At least in theory, the "reduction parameters" between successive drawing blocks can be arbitrarily assigned, the reduction parameters will establish the reductions in cross-sectional areas of the wires and, therefore, the percentages elongation of the wire between successive stages. This will dictate the speeds of the various drawing blocks, bearing in mind that while the physical dimensions of the wire between different drawing blocks changes, the total amount of material remains the same. In practice, the "reduction" parameters are not arbitrarily selected but are fixed by conventions in order to provide standard wire sizes. The standard wire sizes are also a function of the specific metals used to form the wires. For example, in the drawing of copper wires, a standard has been established in the United States designated the B & S American Wire Gauge (AWG). Under this convention, the wire gauges are assigned designations of 6/0 for the largest diameter wire to 56 for the finest wire. The ratio of the diameter of each gauge wire is approximately 0.89 to the diameter of the next or adjacent gauge wire. Having selected, for example, this B & S AWG standard for copper, the "reduction parameters" become defined and, at least in theory, the relative speeds of the various drawing blocks become known parameters. Working with the established ratios of diameters between adjacent or successive B & S AWG gauge wires, it is evident that the wire cross-sectional area must be reduced approximately 20.7% between successive drawing blocks, this resulting in elongations of the wires of approximately 26% per drawing stage. Therefore, in order to compensate for the increased length of the wire between successive drawing blocks, it is equally clear that each successive drawing block must exhibit a linear surface velocity of 126% of the linear surface velocity of the previous drawing block. The B & S AWG wire gauges is but one possible rule of action for a drawing machine. A different set of "reduction" parameters are used when drawing steel wire in accordance with, for example, the W & M steel wire gauge. Still other "reduction" parameters can be used for different materials, for example, aluminum.
In view of the fact that all blocks are usually linked by mechanical means in order to change the final diameter to be drawn, the complete string of dies has to be moved in the machine to modify the final diameter and this results in a big set-up time and loss of production.
With recent advances in electrical speed controlled drives, some wire drawing machines have been designed in such a way as to provide a separate motor and controls for the final block (normally called the "final capstan"). This system allows the user to change the speed of the final capstan in relation to the speed of the main machine, therefore, allowing the operator to match the speed of the final capstan to the next to the last drawing block. This allows the elimination of one die and an increase in the diameter of the final product without restringing. However, such systems reduce the output of the machine since the final speed of the product produced by the die upstream of the removed die is reduced by approximately 26% on a B & S drawing machine. Each time the wire gauge is decreased by one and the wire diameter increases, the linear velocity at the output of the machine decreases by approximately 26% with each removal of another die.
In view of the disadvantage of the aforementioned approach, it has been a common practice not to reduce the speed of the final capstan, but to increase the speed of the main machine to match the speed of the final capstan with the next to the last drawing block. With each next successively larger wire to be produced, (lower AWG gauge), the process is repeated and another drawing die is eliminated and the speed of the main machine is again increased by approximately 26%.
The last mentioned solution can provide some operating advantages, however, it can be immediately seen that this process has obvious limitations. For example, if we consider that the most commonly elongation for non-ferrous metal is 26%, by eliminating only two dies the main speed of the machine would have to be increased by 59%. In view of the fact that in state-of-the art equipment, the last shafts are already turning very close to the limits of existing bearing technology, these speeds cannot be exceeded and the number of diameters that can be produced without restringing is restricted to one or two. In order to avoid this problem, several means have been devised, the most commonly used being the use of clutches on the last shafts of this type of machine. This design allows the uncoupling of these shafts that are not used and would exceed the critical speeds if left connected with the drive system of the main machine, while the main machine speed is increased to match the final capstan surface velocity. Hence, this solution introduces complicated mechanisms and high maintenance items, such as clutches, couplings, etc.